Exploiting Physical Layer Advances in Wireless Networks Michael Honig Department of EECS Northwestern University

IEEE Spectrum Magazine, March 2004

Is spectrum scarce or abundant? Spectrum is scarce –Existing paradigm –Spectral efficiency is an important objective. –Physical layer advances play a crucial role in improving wireless networks. Spectrum is abundant –Does spectral efficiency matter? –How should spectrum be allocated among services (broadcast radio/TV, cellular, internet access,…)? –Should different wireless services share an infrastructure of access points?

Physical Layer Advances Turbo (iterative message passing) MIMO Relay (cooperation)

Adds degrees of freedom (DoFs) for diversity, multiplexing, interference mitigation and avoidance Two approaches: –Transmitters do not acquire Channel State Information (e.g., space-time codes) –Transmitters acquire CSI, optimize resource allocation (e.g., water-filling) Learning CSI can provide substantial benefits –Enables interference avoidance, can simplify coding –MIMO broadcast Overhead (feedback + channel estimation) is probably excessive for mobile users –e.g., 4x4 MIMO OFDM with 10 users, 100 sub-channels… Exploiting MIMO

Exchanging “interference prices” enables distributed power control in a spread spectrum network –Maximizes sum utility over links With MIMO/multi-carrier links, need an interference price for each signal dimension Tradeoff between information exchange (signaling overhead) and performance Transmitter Receiver Distributed Resource Allocation

Exploiting Relays Objective: allocate power/time/bandwidth across links to maximize network objective (e.g., sum utility). –Difficult optimization problem due to half-duplex constraints. Is exchanging local state information (e.g., interference prices) good enough? BST R M R M M M M M resources?

What if spectrum is abundant? Hinges on policy decisions –Spectrum “property rights” vs commons model –Vested interests Still need mechanisms for spectrum sharing –Sensing? –Incentives; spectrum markets? (what’s the commodity?) –How should spectrum be allocated among services? (broadcast radio/TV, cellular, internet access,…) –Should different wireless services share an infrastructure of access points?

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